StackAllocator is not performant and wastes memory on padding

[Monu Yadav]

I assume that the project has been abandoned since a some pull requests are pending.

Anyways, I will address a few things here for those coming to this repository in search of knowledge from this code base.

Problems with using "AllocationHeader"

• Wasted Memory: AllocationHeader as is used in this repository simply adds overhead in terms of wasted padding memory. Regardless of the fact that the memory (m_start_ptr + m_offset) is already aligned, AllocationHeader header wastes as low as alignment bytes or padding + alignment bytes of memory.
• Probable Cache Misses: Since the AllocationHeader is stored before the memory pointer that allocate gives away, there is a probability of cache miss when using free if the memory pulled in does not align with cache boundary. It might be a small cost or a big one depending upon how many times free is called. It's bad since the entire point of a custom allocator is performance with solid metrics of memory usage.
• Current Implementation of AllocationHeader stores garbage: As has been addressed in issue #8, AllocationHeader currently stores garbage since it stores the address of a stack allocated AllocationHeader that would go out of scope once it goes out of scope i.e. as soon as the allocate function ends

AllocationHeader allocationHeader{padding};
AllocationHeader * headerPtr = (AllocationHeader*) headerAddress;
headerPtr = &allocationHeader;   // <---------- allocationHeader would go out of scope

A better way of implementing is to use push/begin constructs that would store the offset of stack and pop/end which would reinstate the stack allocator back to the pushed offset.